/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.instrument.classloading;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

/**
 * Subclass of ShadowingClassLoader that overrides attempts to
 * locate certain files.
 *
 * @author Rod Johnson
 * @author Adrian Colyer
 * @since 2.0
 */
public class ResourceOverridingShadowingClassLoader extends ShadowingClassLoader {

    private static final Enumeration<URL> EMPTY_URL_ENUMERATION = new Enumeration<URL>() {
        @Override
        public boolean hasMoreElements() {
            return false;
        }

        @Override
        public URL nextElement() {
            throw new UnsupportedOperationException("Should not be called. I am empty.");
        }
    };


    /**
     * Key is asked for value: value is actual value
     */
    private Map<String, String> overrides = new HashMap<>();


    /**
     * Create a new ResourceOverridingShadowingClassLoader,
     * decorating the given ClassLoader.
     *
     * @param enclosingClassLoader the ClassLoader to decorate
     */
    public ResourceOverridingShadowingClassLoader(ClassLoader enclosingClassLoader) {
        super(enclosingClassLoader);
    }


    /**
     * Return the resource (if any) at the new path
     * on an attempt to locate a resource at the old path.
     *
     * @param oldPath the path requested
     * @param newPath the actual path to be looked up
     */
    public void override(String oldPath, String newPath) {
        this.overrides.put(oldPath, newPath);
    }

    /**
     * Ensure that a resource with the given path is not found.
     *
     * @param oldPath the path of the resource to hide even if
     *                it exists in the parent ClassLoader
     */
    public void suppress(String oldPath) {
        this.overrides.put(oldPath, null);
    }

    /**
     * Copy all overrides from the given ClassLoader.
     *
     * @param other the other ClassLoader to copy from
     */
    public void copyOverrides(ResourceOverridingShadowingClassLoader other) {
        Assert.notNull(other, "Other ClassLoader must not be null");
        this.overrides.putAll(other.overrides);
    }


    @Override
    public URL getResource(String requestedPath) {
        if (this.overrides.containsKey(requestedPath)) {
            String overriddenPath = this.overrides.get(requestedPath);
            return (overriddenPath != null ? super.getResource(overriddenPath) : null);
        } else {
            return super.getResource(requestedPath);
        }
    }

    @Override
    @Nullable
    public InputStream getResourceAsStream(String requestedPath) {
        if (this.overrides.containsKey(requestedPath)) {
            String overriddenPath = this.overrides.get(requestedPath);
            return (overriddenPath != null ? super.getResourceAsStream(overriddenPath) : null);
        } else {
            return super.getResourceAsStream(requestedPath);
        }
    }

    @Override
    public Enumeration<URL> getResources(String requestedPath) throws IOException {
        if (this.overrides.containsKey(requestedPath)) {
            String overriddenLocation = this.overrides.get(requestedPath);
            return (overriddenLocation != null ?
                    super.getResources(overriddenLocation) : EMPTY_URL_ENUMERATION);
        } else {
            return super.getResources(requestedPath);
        }
    }

}
